manifest: document Pipeline interface

Docstrings for the Manifest.pipelines field and the Pipeline type
itself.

internal/manifest/manifest.go

@@ -51,6 +51,8 @@ func (m *OSBuildManifest) UnmarshalJSON(payload []byte) error {
 // to generate the pipelines but no content. The content included in the
 // Content field must be resolved before serializing.
 type Manifest struct {
+
+	// pipelines describe the build process for an image.
 	pipelines []Pipeline
 
 	// Content for the image that will be built by the Manifest. Each content

internal/manifest/pipeline.go

@@ -9,11 +9,22 @@ import (
 	"github.com/osbuild/osbuild-composer/internal/rpmmd"
 )
 
+// Pipeline serializes to a series of stages that modify a file system tree
+// when used as input to osbuild. Different Pipelines serialize to different
+// sequences of stages depending on their type and configuration.
 type Pipeline interface {
+
+	// Name of the pipeline.
 	Name() string
+
+	// Checkpoint this pipeline when osbuild is called.
 	Checkpoint()
+
+	// Export this tree of this pipeline as an artifact when osbuild is called.
 	Export() *artifact.Artifact
+
 	getCheckpoint() bool
+
 	getExport() bool
 
 	// getBuildPackages returns the list of packages required for the pipeline